Arsenic in drinking water is designated as a priority contaminant in the United States of America under the 1986 Safe Drinking Water Act and amendments thereto. Since 1974, an arsenic Maximum Contaminant Level (MCL) of 50 parts per billion (ppb) has been in effect in the United States. As a result of more recent findings pertaining to health risks associated with populations exposed to high concentrations of arsenic in drinking water, the United States Environmental Protection Agency (EPA) recommends the lowering of the MCL for arsenic from 50 ppb to 2 ppb. In the United States alone, more than 12,000 public water utilities would fail to meet the more stringent proposed arsenic standard. One estimate places the cost of compliance for the 2 ppb MCL proposal in excess of $5 billion/year.
The number of private wells in the United States that fail to meet the existing 50 ppb or proposed 2 ppb MCL for arsenic is unknown. It is believed that in many areas in the USA, many thousands of private wells produce drinking water with potential, serious health risks for the households depending on self-produced water because of arsenic contamination.
Regionally, high arsenic content in drinking water is a global problem. In West Bengal, India, for example, an estimated 200,000 people currently suffer from arsenic-induced skin lesions, some of which have advanced to pre-cancerous hyperkeratoses.
Arsenic is found in several oxidation states. Typically, arsenic is present in aqueous solutions in the oxidation state of plus five (As.sup.+5, pentavalent) and to a lesser extent the oxidation state of plus three (As.sup.+3, trivalent). There is no significant reported cation chemistry for arsenic, but organic arsenic salts are known for both oxidation states (e.g. KAs(C.sub.6 H.sub.4 O.sub.2).sub.2 !).
Examples of trivalent arsenic compounds are the halides (AsCl.sub.3, AsCl.sub.2.sup.+, and AsF.sub.3). The halides are readily hydrolyzed to arsenious acid (H.sub.3 AsO.sub.3) or it acid-dissociated forms (HAsO.sub.2.sup.2-). The oxide form is As.sub.2 O.sub.3. The trivalent arsenic compounds to be separated from aqueous solutions, most likely in an ionized form of H.sub.3 AsO.sub.3, in a process of the invention are collectively referred to herein as "trivalent arsenic".
As.sup.0 can be oxidized by concentrated nitric acid to pentavalent arsenic as arsenic acid (isolable as H.sub.3 AsO.sub.4 .multidot.1/2H.sub.2 O), which is a moderately strong oxidizing agent in solution. The corresponding halides are also known (e.g. AsCl.sub.5, AsCl.sub.4.sup.+). The pentavalent arsenic compounds to be separated from aqueous solutions, most likely an ionized form of H.sub.3 AsO.sub.4, in a process of the invention are collectively referred to herein as "pentavalent arsenic".
Surveys taken of drinking water around the world usually give a total arsenic level and fail to distinguish contributions from pentavalent arsenic or trivalent arsenic, even though trivalent arsenic is considerably more toxic than pentavalent arsenic. The failure to distinguish the valence of arsenic present in drinking water further confuses the logical assignment of MCL values because although a level of 2 ppb of pentavalent arsenic may cause no deleterious health effects, an equivalent level of trivalent arsenic can have negative health consequences.
There is an urgent need for a technology that will remove arsenic from drinking water to provide safe levels regardless of the oxidation state of the arsenic in an efficient, economical and environmentally sound manner. It is desirable that such technology be flexible and sufficiently robust in order to address the requirements of large municipal water utilities, private wells in developed countries and contaminated water sources in undeveloped countries.
A number of technologies have been described in the prior art to remove arsenic from drinking water. These technologies of the art include co-precipitation, alumina adsorption and classical ion-exchange with anion exchange resins. In a report entitled "National Compliance Assessment and Costs for the Regulation of Arsenic in Drinking Water" (January, 1997) prepared by the University of Colorado at Boulder, more than a dozen putative methods are evaluated for arsenic removal efficiency and cost. None of the evaluated methods described exhibited arsenic removal efficiencies greater than 95 percent, nor do the prior art methods offer the simplicity of use required for private well treatment or for underdeveloped areas of the world where reliable electrical power is unavailable.
Moreover, the prior art does not offer an environmentally sound "closure" to the arsenic removal problem. For instance, in situ precipitation with iron hydroxide has been shown to be moderately effective in removing certain arsenic species. A problem that remains from such a technology is the mechanical problem of filtering the iron oxide/arsenic co-precipitate from the water, and ultimately, the problem of disposing of arsenic-laden sludge which can re-leach into the same or different water supply. Much the same criticism can be made of the prior art methods of alum co-precipitation, lime precipitation or alumina adsorption.
Classical ion-exchange with anionic resins of the art suffer from poor efficiency (90 percent), low capacity (1500 bed volumes) and severe reduction in capacity and binding efficiency when competing ions such as sulfate are present in amounts of 50 ppm or more. Classical ion-exchange media suffer from poor longevity when challenged with a matrix of hard well water. It is estimated by the aforementioned Colorado report that 25 percent of such classical resins would have to be replaced on an annual basis.
There remains, therefore, a need for a simple-to-use adsorbent for removing dissolved arsenic from water that exhibits an arsenic removal efficiency greater than 95 percent that is stable and reusable.